I did go ahead and the cake's in the oven. I doubled everything except the baking powder, and I increased the flour to 3 cups. The oven was preheated to 375 and once the cake was in I reduced the temp to 350, so I'm thinking it will bake about 50-60 minutes? I ended up using a large bundt pan.
Will let you know how it turns out--fingers crossed!

It turned out just fine, perfectly done, very moist. No problem turning it out of the pan either. Slices beautifully. I think next time bake at 370 ( took about 70 minutes at 350). Maybe zest only 1 lemon and finely dice the apple? A lovely cake. Thanks for the recipe!